When the tanks of motor vehicles are being filled with liquid fuel delivered from a storage tank, it is known that the fuel expels an equivalent volume of polluting gas to the surrounding atmosphere. Vapor recovery systems are known specifically for returning the polluting gas to the storage tank from which the delivered liquid was taken.
There exist various known methods for recovering these gases that comprise a mixture of air and gaseous hydrocarbons. However, all of them make use of special delivery nozzles. These nozzles contain an opening into which the gases are constrained to enter so as to be conveyed by a pressure difference into a channel that leads to the storage tank for the hydrocarbon liquid.
Whenever it is used, an ideal recovery system would ensure that equal volumes of liquid and of gas are transferred regardless of the flow rate, thereby providing 100% efficiency. In practice, this situation is difficult to achieve given imperfections in control, obstructions, possible leaks in the ducting, and also inevitable misadjustments.
The legislation in force in various countries requires efficiencies to be achieved within set limits, where efficiency is defined as the ratio of the volume of gas recovered divided by the volume of liquid delivered. Failure to recover gas leaves residual pollution, while excess suction gives rise to excess air in the storage tank which in turn leads to a danger of explosion.
In order to guarantee that fuel dispensers are operating within legal limits, an official inspection is required periodically, often once a year.
Until now, no apparatus or system has been available for checking the volume of vapor recovered under such circumstances when delivering fuel.
In other technical fields, it is known that volumes of gas or gaseous volumes can be checked by means of a gas meter (e.g. of the domestic type). It would therefore probably be possible to insert a gas meter in the duct for returning gas to the storage tank either upstream or downstream of the booster pump, thereby making it possible to compare volumes of vapor flow with volumes of delivered liquid fuel.
However, installing a gas meter in existing systems would require considerable work, with the equipment being taken temporarily out of service, thus leading to high costs. The return ducts are often buried or otherwise inaccessible.
Furthermore, there are practical problems that would make it difficult to install such a gas meter for measuring the recovered volume of fuel gases. For example, condensates may still be present in the duct and they could damage or even block the bellows of the meter. Also, it is practically impossible to calibrate a meter that is permanently installed in the duct periodically and easily. Furthermore, the efficiency calculation needs to take account of the pressure differences between the meter and atmospheric pressure.
To remedy the above-specified problems, an object of the invention is to provide a method and apparatus applicable to measuring the efficiency of known "assisted" recovery systems in which pumps and gas flow regulator devices are inserted in the gas return path, while avoiding any interference therewith.